1. Field of the Invention
The present invention generally relates to an optical pick-up apparatus. More specifically, the present invention relates to a diffraction element and an optical pick-up apparatus having the same that has improved tracking control.
2. Description of the Related Art
Optical pick-up apparatuses are employed in devices such as compact disk (CD) players, digital versatile disk (DVD) players, compact disk-read only memory (CD-ROM) drives and so forth to record and/or reproduce information onto or from a disc which is an optical recording medium without contact. To record data, the optical pick-up irradiates a laser beam onto a track on the surface of the disc to form a pit train and record a data bit. To reproduce the recorded data, the optical pick-up optically reads pit data formed on the disc, and outputs an electrical signal corresponding to the recorded data. To this end, the optical pick-up includes a plurality of optical elements, for example, a light diode as a light source for emitting a laser beam, a diffraction element, a beam splitter to adjust the deflection of the laser beam, an object lens for forming an optical path and forming a spot (that is, focusing the laser beam) on the disc, an optical detector for detecting a signal and so on.
The optical pick-up apparatus controls the object lens in the vertical direction, so that a beam spot can be focused on the surface of the disc (this is called a focus control). The optical pick-up apparatus also controls the object lens in the horizontal direction, so that the beam can follow the track (this is called a tracking control). To carry out the focus control and the tracking control, a focus error signal (which will be referred to as a “FE signal”) and a tracking error signal (which will be referred to as a “TE signal”) should be generated. In general, an astigmatic method is used to generate the FE signal. Also, a push pull method (which will be referred to as “PP method”) and a differential push pull method (which will be referred to as “DPP method”) are used to generate the TE signal. In the PP method, a single laser beam is used and whether an optical spot is formed at the center of a track is detected based on the intensity of an incident beam on each area of a photodetecting element that is divided into two areas. As the object lens shifts and tilts for performing tracking servo, a DC offset is caused to the TE signal.
For this reason, the DPP method (also called the 3-beam method) is sometimes used for tracking control. In the DPP method, a laser beam is split into a main beam that is scanned on the center of a track and two sub-beams spaced apart from the main beam by a predetermined distance in the radial and tangential directions respectively, and scanned on the periphery of a groove. The DPP method uses the difference in signals obtained from the three laser beams to correct for the DC offset of the TE signal. When the DPP method is applied to a disc having different track pitches, the sub-beams are not formed on the groove. Therefore, correcting the DC offset of the TE signal in discs having different track pitches remains a problem to be resolved.
Japanese Patent application No. 2004-63073 discloses a method for radiating three beams onto one track to attempt to solve the above-described problem. To give a brief description of the method referring to FIG. 1, a diffraction element 10 is divided into first, second and third areas 12, 14, 16, and the diffraction gratings of each area 12, 14, 16 are dislocated ¼ pitch (P/4) from one another. An incident beam on the diffraction element 10 is diffracted and split into the 0th order main beam MB and the ±1st order sub-beams SB1, SB2. Although the main beam MB does not generate a phase difference, the first and second sub-beams SB1, SB2 generate relative phase differences at −90°, 0°, and 90°, respectively. In each sub-beam SB1, SB2, the (−90) degree phase difference is generated by the first area 12 of the diffraction element 10, the 0 degree phase difference is generated by the second area 14 of the diffraction element 10, and the 90 degree phase difference is generated by the third area 16 of the diffraction element 10.
Referring now to FIG. 2, the main beam MB and the sub-beams SB1, SB2 are reflected from the disc, and diffracted again into three beams (MBa, MBb, MBc), (SB1a, SB1b, SB1c), and (SB2a, SB2b, SB2c), respectively. Especially, the three beams SB1a, SB1b, SB1c diffracted from the first sub-beam SB1 incident on the disc and the three beams SB2a, SB2b, SB2c diffracted from the second sub-beam SB2 incident on the disc coincide with the positions where the phase differences of the first and second sub-beams SB1, SB2 are generated by the first, second and third areas 12, 14, 16 of the diffraction element 10.
As shown in FIG. 2, the diffracted beams (MBa, MBb, MBc), (SB1a, SB1b, SB1c), and (SB2a, SB2b, SB2c) from the main beam MB and the first and second sub-beams SB1, SB2, respectively, interfere with one another, and received by a photodetecting element 22 of the optical detector 20 for use in the main beam and photodetecting elements 24, 26 of the optical detector 20 for use in the sub-beams.
In practice, however, a light beam from the light source sometimes does not pass through the center of the diffraction element because of assembly errors in the diffraction element 10 or other optical elements that occur while assembling the optical pick-up apparatus. In this case, the diffraction element 10 does not generate a phase difference in the main beam MB. Thus, although the light beam emitted from the light source may not pass though the center of the diffraction element 10, the sizes of interfered beams MBb, MBc penetrating a center beam MBa are uniform. As such, an MPP (Main beam Push Pull) signal does not generate an error. On the other hand, in the case of sub-beams SB1, SB2, the interfered beams (SB1b, SB1c) and (SB2a, SB2b) that penetrate the center beams SB1a, SB2a often incline toward one side of the photodetecting elements 24 and 26 due to the above-described assembly errors. This produces errors in SPP1 (Sub beam Push Pull 1), which is calculated using a signal from the first sub-beam use photodetecting element 24, and errors in SPP2 (Sub beam Push Pull 2), which is calculated using a signal from the second sub-beam use photodetecting element 26. Unfortunately, these errors do not cancel each other out if the SPP1 and the SPP2 are added, but rather, increase even more. Eventually, these errors cause an error in the DPP (Differential Push Pull) signal which is the TF signal. This problem gets particularly severe in a DVD-RAM because it has a relatively long distance between a groove and a land.
Accordingly, there is a need for an improved optical apparatus that minimizes tracking errors.